Overview
- Tier 1
- Singles
- Typical difficulty
- Peak Score 1–15
- Also known as
- Sole Candidate, Forced Digit, Singleton
A Naked Single occurs when 8 of the 9 digits are already present in the combined row, column, and 3×3 box seeing a cell, leaving exactly one digit mathematically possible.
The Naked Single is the bedrock of Sudoku solving. It is called 'naked' because once candidates are penciled into the cell, the digit sits completely exposed with no other contenders. While simple in concept, discovering naked singles on an unpenciled board requires scanning all three intersecting houses (row, column, and box) simultaneously. Puzzles in beginner and gentle tiers resolve predominantly through cascades of naked singles.
Worked examples
1 / 5
Reading the exampleCells forming the pattern
47
478
3
2
9
1
5
48
6
6
2
9
5
8
4
3
7
1
5
48
1
3
6
7
9
48
2
2
1
5
8
3
9
7
6
4
3
9
4
1
7
6
2
5
8
8
6
7
4
2
5
1
9
3
1
34
6
7
5
2
8
3
9
9
5
8
6
1
3
4
2
7
47
347
2
9
4
8
6
13
5
Every digit except 6 already appears in row 1, column 9 or the top-right box, so R1C9 must be 6.
Places 6 in R1C9
How to apply it
Spotting the pattern and making the deduction, in order.
- Select an unsolved cell on the grid.
- Check all placed digits along its row, down its column, and inside its 3×3 box.
- If 8 distinct digits are already accounted for across those intersecting units (e.g. 1 through 5, and 7 through 9), the missing number (6) is the only legal option left.
- Confidently place that single remaining number into the cell.
Common mistakes
- Overlooking one of the 3 intersecting houses—often checking the row and column carefully but forgetting a digit already placed in the opposite corner of the 3×3 box.
- Confusing a Naked Single with a Hidden Single: a Naked Single is forced because the *cell* has only one choice; a Hidden Single is forced because the *house* has only one place for a digit.
- Prematurely placing a digit based on intuition without verifying that all other 8 digits are genuinely accounted for.
Frequently asked questions
- What is the difference between a Naked Single and a Hidden Single?
- A Naked Single means a specific cell has only one possible candidate remaining across its row, column, and box. A Hidden Single means a digit has only one legal cell inside an entire row, column, or box, even though that cell might still hold multiple other candidates.
- Do I need full pencil marks to find Naked Singles?
- Not necessarily. Skilled solvers find naked singles on unpenciled boards by scanning cells whose row, column, and box share many placed digits. However, writing candidate pencil marks makes naked singles immediately obvious as cells with only one number.
- Can an expert Sudoku puzzle contain Naked Singles?
- Yes! Even the most diabolical puzzles often finish with a cascade of naked singles once an advanced technique (like an X-Wing or ALS Chain) makes key eliminations and cracks the puzzle open.
Related techniques
Put Naked Single into practice
Try a puzzle from the Beginner tier (Peak Score 1–10).